No need for a new tube,

Just use a drill bit up the bottom of the FSB bottom hole one you have the FSB with gas tube off the barrel (the one that was used to drill the gas port passage in the FSB to begin with) and enlarge the tube end of passage hole to match the FSB passage channel size (read don't even need to pull the gas tube out of the FSB).  IF you have a gas block, then remove the back set screw once you have the block/tube off the barrel since it was the channel used to drill the gas passage as well.